Mucin 1 (MUC1) is aberrantly overexpressed in ~90% of human breast cancers, including the triple-negative (TNBC) subtype, and is associated with poor progression-free and overall survival. MUC1 has thus emerged as a highly attractive target for the treatment of TNBC; however, to date there are no approved agents against this heterodimeric transmembrane protein. What is needed now, at least in part, is therefore the development of novel agents that target MUC1-C for the treatment of patients with refractory TNBC. Importantly, MUC1 consists of two subunits: an extracellular N-terminal mucin subunit (MUC1-N) that is shed from the cell surface, and a transmembrane C-terminal subunit (MUC1-C) that is oncogenic. MUC1-C functions as an oncoprotein by acting as a node for integrating signaling pathways linked to transformation. In this way, MUC1-C drives (i) the epithelial-mesenchymal transition (EMT), (ii) self-renewal capacity, and (iii) tumorigenicity of TNBC cells. Early attempts at targeting MUC1-N with monoclonal antibodies (MAbs) were unsuccessful because this shed subunit circulates at high levels in the blood of women with breast cancer. As an alternative approach, we have generated novel MAbs against the non-shed MUC1-C extracellular domain. These MAbs have provided a unique opportunity to develop antibody-drug conjugates (ADCs) that specifically target MUC1-C on the surface of breast cancer cells. Our MAbs are also being advanced for the development of antibody-dependent cell-mediated cytotoxicity (ADCC) and for combinations with other immunotherapies. Recent Phase I trials using single-agent anti-PD-1 and anti-PD-L1 blockade have demonstrated objective and durable responses in heavily pretreated, metastatic TNBC, supporting the premise that evasion of immune destruction contributes to TNBC pathogenesis. Our hypothesis is that our ADCs will be effective against MUC1-expressing breast tumors in our immunocompetent mouse models and that killing these tumor cells could reverse, at least in part, the immune suppressive effects of the tumor microenvironment. In this R21 application, our immunotherapeutic approaches targeting MUC1-C will be developed for clinical evaluation based on their effectiveness against TNBC cells growing in vitro and in animal models. The overall objective of the proposed work is thus to improve the immunotherapy of breast cancer. Our hypothesis is that MUC1-C is an attractive target and that our immunotherapeutic approaches can be developed for patients with TNBC.